1. Field of the Invention
The present invention relates to a solid oxide fuel cell in which a plurality of unit elements are formed on a support tube.
This application is based on Japanese Patent Application No. 2009-145015, the content of which is incorporated herein by reference.
2. Description of Related Art
In a typical structure for a cylindrical solid oxide fuel cell (SOFC), a plurality of unit elements are formed along the lengthwise direction of a support tube, and adjacent unit elements are linked via an interconnector. The term “unit element” describes an element prepared by sequentially stacking an anode, an electrolyte and a cathode on a porous support tube. Although the voltage from a unit element is small, the total voltage is increased by connecting a plurality of elements in series, thus generating a high output.
An interconnector is a material that electrically connects the unit elements. If the conductivity of the interconnector is low, then the extractable electrical power level decreases, and therefore the interconnector requires a high degree of conductivity. Generally, a LaCrO3-based material is used as the interconnector material (for example, see Japanese Unexamined Patent Application, Publication No. Hei 09-263961). Further, the interconnector also performs the role of preventing mixing of the fuel and the air when a fuel gas is supplied to the interior of the support tube. Accordingly, a high level of gas tightness is required at the interface between the interconnector and the unit elements.
The LaCrO3-based material disclosed in Japanese Unexamined Patent Application, Publication No. Hei 09-263961 exhibits poor sinterability, and therefore not only must a high firing temperature be employed, but the resulting sintered compact suffers from a low degree of densification. When producing a solid oxide fuel cell using a LaCrO3-based material, because the firing temperature must be raised, the support tube undergoes densification. As a result, the support tube loses the ability to allow passage of the fuel, causing a deterioration in the output properties of the solid oxide fuel cell. On the other hand, if the firing temperature is kept low in order to prevent the densification of the support tube, then the densification of the LaCrO3-based interconnector deteriorates. As a result, when a solid oxide fuel cell is produced, the fuel tends to leak through the interconnector, causing a reduction in the electric power generation efficiency of the fuel cell. Furthermore, when N2 purging is performed in the case of an emergency, oxygen may penetrate through to the anode, causing oxidation of the anode that will result in cracking or the like.
In order to address these problems, other methods of producing the interconnector for a solid oxide fuel cell besides the co-sintering method mentioned above have been proposed, including electrochemical vapor deposition (EVD) methods and spraying methods, but both these types of methods result in increased production costs.
In a unit cell for a solid oxide fuel cell, if there is a large difference in the coefficients of thermal expansion for the interconnector material and the materials used for each of the layers within the unit elements, then tensile stress and/or compressive stress occurs at the contact interface between the interconnector and each of the layers of the unit elements, producing strain and worsening the gas tightness. For example, if fine gaps exist at the contact interface between the electrolyte and the interconnector, then this can cause a reduction in the electric power generation efficiency as a result of fuel leakage, and cracking or the like as a result of anode oxidation.